Can't read mails from russia

Hello,

using the web client (Zimbra 3.1.1 on Debian Sarge, Firefox 1.5.0.3 on Windows XP SP 2) I can't read mails from Russia. The mails usually have the encoding "windows-1251" which is correctly set in the header of the mail nut Zimbra shows me garbage. What can I do to read these mails using the Zimbra web client?